1. Field of the Invention
This invention relates to charged particle beams, and more specifically to a magnetic lens and deflector for deflecting such beams.
2. Description of the Prior Art
Electron beam lithography, electron beam inspection systems and scanning electron microscopes usually use a magnetic objective "lens" to focus a (charged) particle beam, and magnetic deflection coils mounted within the lens to accurately position the focused beam upon a substrate.
The magnetic properties of the deflection optics are critical for achieving fast and accurate beam placement. In particular, the relationship between focusing magnetic field of the lens and the deflection field must be optimized for minimum deflection aberrations and normal landing angle. The return magnetic flux from the deflection coils must be confined within a magnetically soft, high resistivity low hysteresis enclosure that discourages eddy current effects that can cause the beam to settle slowly after a change in deflection current. The charged particles are e.g. electrons and ions. "Soft" in the context of magnetism means low remanence (low coercive force). "Hard" means the opposite. Thus a permanent magnet is of a magnetically hard material.
Prior art magnetic objective lens systems enclose the deflection coils in cylindrical shaped outer return yokes or a series of rings to confine the deflection flux. These yokes or rings often serve as concurrently as pole pieces for the static magnetic lens field generated by a solenoidal lens excitation coil located outside the pole piece structure. In this overlaid structure the focussing magnetic flux flows in axially symmetric paths while the magnetic flux from the deflection coils follows an azimuthal path from one side of the pole pieces to the other. Thus gaps that provide a focusing field on the lens axis can still provide a continuous return path for the deflection flux.
The pole pieces surrounding the deflection system must be of a magnetically very soft material with minimum hysteresis, since otherwise the relationship between deflection coil current and deflected beam position depends upon past deflection history. In addition, the deflection system enclosure must confine all deflection flux, for otherwise flux leakage causes eddy currents in surrounding material. These eddy currents can generate opposing magnetic fields that would temporarily influence the flux distribution, causing the deflection to settle more slowly than desired. (Rapid changes in deflection are advantageous.)
To avoid saturation effects due to axial lens fields, a composite concentric-gap magnetic lens was earlier disclosed by the present inventors; see U.S. Pat. No. 4,469,948 issued Sept. 4, 1984 showing use of one or more sets of cylindrical inner pole pieces, made of a magnetically soft material such as ferrite, arranged concentrically within a magnetic lens circuit excited by a solenoidal excitation coil and whose outer pole pieces are made of a material that supports a higher saturation flux. Magnetic flux is shared between the inner and outer pole pieces in such a way that saturation of the inner pole piece is avoided because excess flux is shunted through the outer pole piece. This assures a linear relationship between the axial lens field and the excitation current at all points along the beam path, which is essential for optimum deflection optics. As is now understood by us to be equally important, but not emphasized in that disclosure, the concentric geometry assures that the magnetic permeability of the defection return path formed by the inner pole pieces remains high. This minimizes settling effects that would otherwise occur if flux escaped the inner enclosure. The net result is a more linear relationship between deflection current and beam position in a high bandwidth deflection system.
Present FIG. 1 is the structure disclosed in U.S. Pat. No. 4,469,948. A deflection coil 1 is enclosed within cylindrical inner pole pieces 2, 3 of a magnetically soft material such as ferrite. Pole pieces 2, 3 define a lens gap 4 that creates a focusing field along the lens axis. Surrounding this inner enclosure is an outer lens circuit excited by a solenoid excitation coil 5 and made of a higher saturation material to form a second set of outer concentric pole piece elements 6, 7. To avoid sharing of deflection flux, the inner pole piece structure is separated by a gap 8 at either end from the outer pole pieces 7. Composite lenses of this type have been used successfully for many years.
More advanced particle beam optics applications will require stronger lens fields and even faster and more accurate deflection. Higher beam energies require stronger lens fields that could exaggerate saturation effects. Improved deflection optics favors multi-stage deflection coils placed in strategic locations along the beam path. In particular, it has been found to be desirable to place at least one deflection coil near the exit of the lens, downstream of the maximum lens field. These new optimization requirements suggest modifications to the original lens geometry.
The lateral position of the inner pole pieces of a magnetic lens determines its electron optical center. It is critical this center not move laterally, because this leads to uncontrolled drift of the beam position. Lens and deflection coils generate heat which leads to temperature changes within the lens, so lateral movement of the lens center due to thermal expansion can be a problem. Features of the lens circuit design can alleviate this problem.
Hence there is a need to improve a composite concentric-gap magnetic lens and deflection system, providing a more effective deflection geometry while still avoiding saturation and flux leakage effects, and to minimize thermal expansion drift.